IOL polymers can be broadly categorized into two groups: (1) materials that absorb less than 1 percent water and (2) materials that absorb more than 1 percent water. Materials that absorb less than one percent water are typically referred to as hydrophobic polymers. Hydrophobic polymers may be foldable at room temperature. Their “foldability” results from their composition rather than from water acting as a plasticizer. Water absorbing polymers are typically referred to as hydrophilic polymers or hydrogels. The most common materials in this group have approximately 25 percent water by weight. Hydrophilic polymers are usually foldable at room temperature by virtue of absorbed water acting as a plasticizer.
The conventional thinking in the intraocular lens (IOL) industry is that hydrophilic IOLs must be immersed in water or saline during storage to maintain a level of hydration needed for foldability at room temperature. Accordingly, lenses composed of these materials are almost always packaged in normal saline (0.9 percent sodium chloride). Such lenses may reside in saline for up to five years prior to implantation. The normal saline, in which these lenses are packaged, is generally designed to mimic the conditions of the anterior segment of the eye where the lens will reside following implantation. This means that lenses will have similar dimension and mechanical characteristics in the eye as they have in the package where they reside prior to implantation.
Immersion in water or saline in the presence of other plastics needed for retention or insertion of the IOL, however, can result in contamination of the IOL by chemical entities contained in or produced by other plastic components in the packaging system. Being immersed in water or saline, the IOL can “communicate” with plastic components via the liquid phase. Also, when an insertion instrument is removed from the package for use, water can flow from where the instrument was stored within the package to the IOL.
The packaging of IOLs in water or saline has the additional disadvantage, especially when the IOL is packaged with an insertion instrument, of increasing the weight of the package, thereby increasing shipping costs.